1. Field of the Invention
The invention is related to digital imaging. More specifically, the invention relates to architectures and methods for the automatic focusing of digital imaging devices.
2. Description of Related Art
An imaging device such as a digital camera consists of, among others, a lensing sub-system and an image processing sub-system. The lensing system focuses upon an external scene and allows light (illuminant) therefrom in a specified bounded area to strike a sensor array. The sensor array consists of photo-cells which when struck by the illuminant within a particular absorption period will accumulate charge. This charge is then converted into a digital value (pixel value). These captured pixels are then subjected to image processing techniques such as compression, scaling image enhancement, etc. Processed images that are stored in a memory device can then be downloaded into a camera or removed (in the case of a removable memory such as a FLASH card being employed in the camera) for use.
The focusing of the lens system within the camera can have an impact upon the fuzziness or sharpness of the scene as represented by the pixels of the sensor array. Focusing may be cone manually, but is often done automatically. One such automatic focusing technique relies upon FIR (Finite Impulse Response) filters to determine where edge features lie and with what certainty edges can be determined. In images that have high entropy, i.e., pixels of widely varying intensity/color values, such as technique is less effective since averaging yields distorted results. Thus, a focusing technique is needed that can better determine the sharpness of the image regardless of its entropy.
Furthermore, with regard to implementation, if focusing is to be implemented in hardware such as a CMOS (Complementary Metal-Oxide Semiconductor) imaging device, it is important to reduce the computational complexity of the focusing procedure, especially when many other functions must also be carried out by the device. With devices that are dual-moded, which may provide both motion and still imaging, there is also desired methods and apparatus that can readily provide different levels of focusing.
In a device such as a digital camera, whether still or motion video, automatic focusing should be able to be performed in integrated circuits with a minimum of cost and area. Ideally, the focusing method would be able to utilize components used elsewhere in the imaging device for other purposes without building special circuitry for automatic focusing itself.